External Chain Plates

[hilbert]

I've received a couple private messages about the external chain plates that I fashioned for my CD28.
There is already a lot of good information that has been posted. Try doing a search for chainplates and you will find links like:
http://www.capedory.org/board/viewtopic ... al#p127877 and more pictures of Fred's chainplates can be found at:
http://www.sbastro.com/FeNIX/mainFrameSet.htm See Projects/Interior/Salon and Projects/System/Rig.


Paul Calder had some interesting thoughts about what material to use:
http://www.sailfeed.com/diy-chainplates ... -materials and blogs about his experience:
http://www.sailfeed.com/diy-chainplates ... -stainless and http://www.sailfeed.com/installing-chainplates

I modeled my chainplates after Fred's posts, with advise from DaveCD28.

• For the external chainplates, I used 316 bar purchased online (take a look at speedymetals.com). The back-stay and upper shrouds were 3/8" thick x 1 1/2" wide and the lower shrouds were 1/4" thick (I will need to measure the actual lengths as I may have cut them). For the backing plates, I used 304 bar, 1/4" thick.
  I cut the holes with a small bench press, cobalt bits and lots of cutting oil. The bolts are A2, 304.
  I positioned all the eye holes to line up with the original eye pads.
  I made a frame to bend the chainplates for the backstay and lower shrouds with a hydrolic jack. The backstay needs a bend of about 45 degrees and the lower stays may have been in the neighborhood of 15 degrees? It helps to attach the chainplate with one bolt and then tighten up on the shroud to find the proper angle fore and aft.
  I used an oscillating multifunction tool to cut the fiberglass off the rebar and an angle grinder with a 4 1/2" cutting disk to cut the rebar and then a 60 grit flap sanding wheel on the angle grinder to clean it all up.
  I added reinforcing fiberglass and bedded the backing plates in thickened epoxy with chopped strands of fiberglass.
  The dust and epoxy fumes are pretty noxious. I now use a full face respirator when I do any similar work.
  I haven't yet decided if I want to make all the standing rigging uniform with 1/4" wire or keep 3/16" lower shrouds. I'll make a decision in the fall/spring when I lower the mast.
  At first, I just cut notches in teak for the chainplates. I later discovered that the deck had deformed and cracked above the old backing plates. When the standing rigging is out of the way, I will fiberglass over the hull deck joint in the area of the chainplates.
  I still need to "polish" the plates (another winter project).

Here are some pictures:

Chainplate on transom:

Backing plate bedded in thickend epoxy:

Top sides:

I will glass over the joint here as well:

Bending the bars:

[Russell]

What material did you use for the backing plates?

[hilbert]

For the backing plates, I ordered 1/4" x 1 1/2" 304 stainless bars. The bars that I received were 1/4" wider than my order.

While I used 316 for the external bars, I reasoned that the 304 backing plates were in a much friendlier environment and worked only in compression.
It seemed reasonable to me that if the original mild steel backing plates lasted 35+ years, then these would do fine. I considered using 6061 aluminum, but I prefer to avoid the galvanic corrosion. The bolts are also 304.

The reason that I didn't use all 316, including A4 316 fasteners, is that I am working on a large number of projects and I need to compromise on where I apply limited resources.

[Adamhagan]

Jonathan,

Thank you for this compilation, this will be my winter project and you have saved me lots of time.

[Russell]

How much additional fiberglassing did you do before installing? What was the resulting hull thickness in the area when done?

I plan to move to external chainplates when I replace my standing rigging, in the next 2 years. After seeing a boat nearly dismasted that literally had a chainplate bolts rip up through the glass, I have been thinking of how to seriously beef up the areas where the chainplates will be thru bolted. A friend of mine who just did this project did several layers of glass, then 3/4 mahogony then the backing plate on top of that. While mahogony is a good rot resistant wood, I think I will go with some sort of composite board like cooza that is rot free and will glass it in entirely.

[Steve Laume]

Why are people going to external chain plates? Are there problems with your existing chain plates or the backing plates? Deck core issues? Just seems stronger? Are four bolts in shear stronger than two in tension? Easier to inspect? What is the weak link in the shroud and stay system?

The original design is certainly clean and appears to be strong and effective. This is definitely something you want to get right is you are going to change things, Steve.

[Adamhagan]

I have yet to meet a Cape Dory that does not have extensive corosion of the mild steel chainplates. It was a poor design material, impossible to inspect and maintain.

Albeit a different design issue, I was crew on a J24 race where our boat had the starboard plate tear through the deck. Fortunately no dismasting and only minor injuries.

I have not heard of any reports of failure but my port side has lost its structural integrity.

[Russell]

I am not fond of the origonal design. I am lucky enough to at least have the aluminium backing plates rather then mild steel, but still. Lets see, bronze top, attached via SS bolts to a aluminium backing plate, attached to bulkhead with SS strap. How many mixed metals can we acheive in a chainplate system? The system is also very difficult to visually inspect and such a crucial system in my opinion should be entirely accessible and easy to inspect.

I understand the need on beamy modern designs for internal chainplates, but on our narrow beam cape dorys it serves no purpose other then cosmetic. I honestly think external chainplates look good, salty and functional.

They are far more practical, you spread the loads much wider, everything you need to see is generally right in front of you and if you design everything well, removal for inspection and rebedding is very easy.

The biggest problem is that since the hull was not designed for external strap chainplates, is the concern that the thickness in that part of the hull wont be up to the job, so I intend to put a lot of focus on seriously beefing it up. I also think I will make the internal backing plate an L shape, so it butts up against the hull/deck joint where the origonal chainplates were, to help transfer some of the load there.

Personally I am not going to mess with the chainplate for the forestay and inner though, the idea of pulling out that plate and making a new one makes my head hurt and mine is in good condition (I inspected it closely when replacing the bowsprit).

[Ron M.]

Russell,
consider FRP,(fiber re-inforced polymer) board to build up your hull thickness for external chain plates. It can be glassed right in and is strong stuff.
Available from mcmaster-carr, manhattan supply and others in various sizes and thicknesses.
I have used it for thru hull backing plates and glassed to engine stringer modification for a re-power.
Not sure how much additional thickness you'd require 1/4" - 3/8 " ?
Access to that area may be problematic but doable .

[Russell]

Ron,

Coosa, which I intend to use, is similar what you describe. Its a fiber reinforced high density closed cell polyeuthane foam. Its often used for bulkheads and powerboat transoms where large outboards are used. I used it to make my subfloor when I built the new cabin sole and its easy to work with, super strong and rot free.

[hilbert]

As for additional fiberglass I used a layer of mat to fill the pockets and then
I added three additional alternating layers of mat and bi-axial for the uppers and back-stay (ave. thickness ~ 3/4")
and three or four layers of bi-axial for the lowers (ave. thickness ~ 9/16)
with Clark Crafts Epoxy plus. It withstands flexing and impacts better than harder, more brittle epoxies (also no amine blush, 1 to 1 mix ratio).

From Fred http://www.capedory.org/board/viewtopic ... al#p127877

For extra strength I added three layers of bi-axial f/g using vinyl-ester resin. This step would be optional for most people.

Paul Calder is more laid back http://www.sailfeed.com/installing-chainplates

Each bolt has a stainless finder washer to distribute the load against the plywood backing plates. Stainless backing plates would be better than plywood but the hull is so thick here that as long as they stay dry this should be ok. As the chainplates are fully external it will be easy enough to beef up the backing if the plywood starts to crush under the load.

[Kevin Kaldenbach]

I would have to disagree with those who think traditional chain plates are better then the typical Cape Dory system. Except for the problems associated with mild steal I think the set up is the cats meow. The problem with the traditional set up is that as loads shift so does the angle that the load exerts on the chain plate. This bends the chain plate similar to bending a coat hanger back and forth until it snaps. Not including the issues of mild steal I would like to know the failure rate of the two different designs.

[Steve Laume]

I suppose there is the definite possibility of the mild steel backing plates rusting away to the point that the bolts pull right through the deck. Has this ever happened to any Cape Dory that anyone has owned or known of? This would of course be a lack of diligence or the impossibility of proper inspection.

Raven has aluminum backing plates. I look at them on occasion and all seems well with no water seepage. I do wonder if I should have replaced the bolts and what should be used.

I might consider changing the back stay as it would not be very difficult to do.

I really like the clean lines and water flow over the top sides that the original set up provides. The external chain plates do look heavy duty but I am not convinced it would be an improvement in strength. They must be a PIA to buff and wax around, Steve.

[hilbert]

It comes down to person preference. It is hard to fault a system that has worked for decades. However on my boat, the plates began to warp the deck and crazing was visible on the topside. I am confident that even with all the deficiencies involved in fabricating my own chainplates, they will survive me. There are toggles to deal with changing angles and how is it any different from the tang on the other end?

Steve, is not Raven a more suitable name for a Baltimore boat?

Crazing is visible between 11" and 12". Either from the expanding steel and rust or frozen water in the pocket or both.


Toggle:

[Kevin Kaldenbach]

First let me say that it is not my point to prove someone wrong at any cost so take this as you may. Second, if I had rotten mild steal in my boats chain plates I might go to externals if it is was too difficult to go back to original but I would consider that I settled for the second best choice.

As far as toggles they will not change the direction that force is put on your chain plates they will just relieve un even strains at the connection. Don’t believe me? Hook a tow chain to a metal fence post and pull it out. It will most likely bend the post at every angle except pulling straight up. Even though links of the tow chain are a bunch of toggles in a row.

I doubt if the crazing you showed has any thing to do with your chain plates. That circular pattern is most likely from an impact. Now if you showed me crazing on the deck in the general pattern of chain plate under it you might just have showed me the light and made me a believer.

We all love our Cape Dory’s and most have great admiration for Carl Alberg and his ability to design a fine boat. If we were to believe the riggers who tell us that our rigging is too small, or the surveyor who comments of the crazing in the gel coat, or any of the other host of critiques that we get from arm chair naval architects we might just as well be sailing Benahunalinas.